1. Field of the Invention
This invention relates generally to friction stir welding (FSW) and its variations including friction stir processing (FSP), friction stir spot welding (FSSW) and friction stir mixing (FSM) (and hereinafter referred to collectively as “friction stir welding”). Specifically, the invention relates to the problems that are unique to friction stir welding of high temperature materials when these materials are in the shape and size of a small pipe or tubular object (hereinafter “tube, tubes, or tubing”). The specific focus of the inventions are 1) holding a small diameter tube in order to perform friction stir welding, 2) preventing deformation of the small diameter tube caused by the force of a FSW tool by providing an internal mandrel, 3) providing a FSW tool that has a geometry that is uniquely adapted to performing FSW on a small diameter tube, and 4) the process parameters that must be used when performing FSW on a small diameter tube.
2. Description of Related Art
Friction stir welding is a technology that has been developed for welding metals and metal alloys. Friction stir welding is generally a solid state process. Solid state processing is defined herein as a temporary transformation into a plasticized state that typically does not include a liquid phase. However, it is noted that some embodiments allow one or more elements to pass through a liquid phase, and still obtain the benefits of the present invention.
The friction stir welding process often involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin. Force is exerted to urge the pin and the workpieces together and frictional heating caused by the interaction between the pin, shoulder and the workpieces results in plasticization of the material on either side of the joint. The pin and shoulder combination or “FSW tip” is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing FSW tip cools to form a weld. The FSW tip can also be a tool without a pin so that the shoulder is processing another material through FSP.
FIG. 1 is a perspective view of a tool being used for friction stir welding that is characterized by a generally cylindrical tool 10 having a shank 8, a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a workpiece 16 until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized workpiece material. Typically, the pin 14 is plunged into the workpiece 16 until reaching the shoulder 12 which prevents further penetration into the workpiece. The workpiece 16 is often two sheets or plates of material that are butted together at a joint line 18. In this example, the pin 14 is plunged into the workpiece 16 at the joint line 18.
FIG. 2 is a cross-sectional view of the tool 10. A collar 22 is shown gripping both the shank 8 and the FSW tip 24, wherein the FSW tip is comprised of the shoulder 12 and the pin 14. As the tool 10 is rotated, torque is transmitted from the rotating shank 8 to the collar 22 and then to the FSW tip 24. When the tool 10 is being used on a workpiece that is a high melting (or softening) temperature material such as steel, the FSW tip 24 is in many situations exposed to temperatures in excess of 1000 degrees C. as it is rotated while traversing steel softened by frictional heating.
Referring to FIG. 1, the frictional heat caused by rotational motion of the pin 14 against the workpiece material 16 causes the workpiece material to soften without reaching a melting point. The tool 10 is moved transversely along the joint line 18, thereby creating a weld as the plasticized material flows around the pin from a leading edge to a trailing edge along a tool path 20. The result is a solid phase bond at the joint line 18 along the tool path 20 that may be generally indistinguishable from the workpiece material 16, in contrast to the welds produced when using conventional noon-FSW welding technologies.
It is observed that when the shoulder 12 contacts the surface of the workpieces, its rotation creates additional frictional heat that plasticizes a larger cylindrical column of material around the inserted pin 14. The shoulder 12 provides a forging force that contains the upward metal flow caused by the tool pin 14.
During friction stir welding, the area to be welded and the tool are moved relative to each other such that the tool traverses a desired length of the weld joint at a tool/workpiece interface. The rotating friction stir welding tool 10 provides a continual hot working action, plasticizing metal within a narrow zone as it moves transversely along the base metal, while transporting metal from the leading edge of the pin 14 to its trailing edge. As the weld zone cools, there is typically no solidification as no liquid is created as the tool 10 passes. It is often the case, but not always, that the resulting weld is a defect-free, re-crystallized, fine grain microstructure formed in the area of the weld.
Travel speeds are typically 10 to 500 mm/min with rotation rates of 200 to 2000 rpm. Temperatures reached are usually close to, but below, solidus temperatures. Friction stir welding parameters are a function of a material's thermal properties, high temperature flow stress and penetration depth.
Previous patents have taught the benefits of being able to perform friction stir welding with materials that were previously considered to be functionally unweldable. Some of these materials are non-fusion weldable, or just difficult to weld at all. These materials include, for example, metal matrix composites, ferrous alloys such as steel and stainless steel, and non-ferrous materials. Another class of materials that were also able to take advantage of friction stir welding is the superalloys. Superalloys can be materials having a higher melting temperature bronze or aluminum, and may have other elements mixed in as well. Some examples of superalloys are nickel, iron-nickel, and cobalt-based alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium.
It is noted that titanium is also a desirable material to use for friction stir welding. Titanium is a non-ferrous material, but has a higher melting point than other nonferrous materials. The previous patents teach that a tool for friction stir welding of high temperature materials is made of a material or materials that have a higher melting temperature than the material being friction stir welded. In some embodiments, a superabrasive was used in the tool, sometimes as a coating.
The embodiments of the present invention are generally concerned with these functionally unweldable materials, as well as the superalloys, and are hereinafter referred to as “high melting temperature” or “high softening temperature” materials throughout this document. Nevertheless, the tool to be taught herein can also be used in less harsh friction stir welding environments when low melting temperature materials are being used.
The present invention is useful for tools being used in many applications, but most especially when performing friction stir processing of high melting temperature materials.
Small diameter tubes (including casing and pipes) often require mechanically fastened connectors, often made of dissimilar materials, to maintain a seal over a wide temperature range. As a result, the joint is prone to leaking due to extreme temperature gradients as the two materials expand or contract at different rates. Tubular systems that transport liquid natural gas are one example of mechanical components that must function over an extreme temperature range.